Histone deacetylases (HDACs) are a class of enzymes, e.g., HDAC1-11, that regulate gene expression via histone acetylation. HDAC inhibitors have been known to induce expression of p21 and/or p27, two proteins that induce cell growth arrest, differentiation, and apoptosis in tumor cells. See Nimmanapalli et al., Blood 2003, 101, 3236-39; and Roy et al., Molecular Cancer Therapeutics 2007, 6, 2696-2707. They have thus attracted great attention as potent anticancer agents. See, e.g., Lu et al., Journal of Medicinal Chemistry 2005, 48, 5530-35; Kulp et al., Clinical Cancer Research 2006, 12, 5199-5206; and Ryan et al., Journal of Clinical Oncology 2005, 23, 3912-22.